1. Field of the Invention
This invention relates to the field of caulk guns, and more particularly relates to dripless caulk guns.
2. Description of the Related Art
Caulk guns are used for dispensing and applying caulk. Caulk is used by building contractors and homeowners for sealing windows and doors, particularly as a means of weatherproofing and sealing out outdoor weather and air. One industry convention for dispensing caulk comprises a caulk gun in combination with a plastic or cardboard cylindrical, caulk-filled canister or tube. One end of the canister is provided with a nipple or nozzle for dispensing the caulk. Another end of the canister is provided with a drive plate (a movable end plate within the caulk tube) that rides along an interior surface of the canister and pushes against the caulk so as to dispense caulk through the nipple.
Prior art caulk guns provide a body, that receives and holds the caulk canister, a trigger and a plunger mechanism including a piston for pressing against the drive plate in the caulk canister. Many caulk guns provide a ratchet and pawl arrangement wherein the trigger may be squeezed to advance the plunger and, as the trigger is released, the pawl clicks backward along one or more notches in the plunger so that when the trigger is squeezed again it advances the plunger from its previous position. With the ratchet and pawl engaged, the plunger is not free to move backwards. To release the plunger and move it back from the canister, the plunger must be rotated to disengage the pawl from the notches in the plunger, thereby permitting the plunger to move freely backwards. One disadvantage with the ratchet and pawl system is that it drips caulk even when the trigger is released because pressure that has been built up within the canister continues to force caulk out until that pressure has been relieved and equalized with atmospheric pressure. This is a substantial drawback of the conventional caulk gun because it interferes with applying the caulk neatly and can lead to excess caulk dripping onto otherwise clean surfaces.
Another type of caulk gun, referred to herein as a biased-spring drive system, is shown in Chang, U.S. Pat. No. 4,081,112. Chang discloses a biased "plunger drive grip" that grabs a plunger rod when it is canted by a trigger. Further motion of the trigger moves the drive grip and plunger against the caulk gun for dispensing caulk. Chang provides a second trigger for releasing the plunger so it can move back to take pressure off the caulk tube. Thus, the caulk gun of Chang would drip excess caulk until the second trigger is depressed to relieve pressure in the caulk tube.
Various references disclose prior art attempts to solve the problem of dripping caulk. Finnegan, U.S. Pat. No. 4,461,407 discloses an automatic pressure release mechanism which is mounted onto a plunger rod. The release device is essentially a resilient O-ring. The O-ring is loose enough so that the rod can slide through it but tight enough so that as trigger pressure on the plunger rod is released, the resilient memory of the O-ring will cause the plunger rod to move backward slightly, thereby releasing the pressure of the plunger on the caulk canister. As is readily apparent, a close tolerance between the O-ring and plunger is required which increases the cost of the caulk gun and provides a ready mode of failure as the O-ring wears.